1. Field of the Invention
The invention relates to the field of reactive polyurethane flows. More particularly, it relates to container modifications that reduce defects in polyurethanes formed by flowing reactants into the containers.
2. Background of the Art
Polyurethanes are widely employed to fill spaces and serve other purposes in structures. Examples may include their use in appliance walls, where they may contribute to provide insulation against heat or cold as well as structural strength; in automobiles, where they may provide insulation against noise, vibration, and/or fire; and in ship or boat hulls, where they may provide buoyancy. For these and other applications, it is common to introduce the polyurethane formulation's components into the space, for example, by injection thereof, and allow the components to react to form the final polyurethane therein. This method enables the polyurethane to simultaneously fill and take the desired final shape of the space. It is also often desirable that the polyurethane be as uniform as possible, having few and/or small voids therein, such that any of the desired roles of the polyurethane (e.g., structural strength, buoyancy, and/or insulation against heat, cold, noise, etc.) are not compromised.
Unfortunately, the processes by which the polyurethane is formed, e.g., injection, exerts or results in flow, of the components and/or of the forming (reacting) polyurethane, prior to the setting and subsequent curing thereof. During this time, and depending upon the viscosities of the flowing material(s), the overall reactivity of the formulation, and the method and rate of introduction of the components, there is often a tendency for voids to form. For example, use of vacuum-assisted injection methods (pressures of, e.g., 0.6 to 0.8 bar, 60 to 80 kPa) along with highly reactive foamed polyurethane formulations will usually greatly increase this tendency, because the shear stresses exerted along the walls of the container into which the components are introduced result in some destabilization of the forming foam. Voids include a variety of holes within and/or at the surface of the foam that differ from the overall desired structure of the polyurethane. Thus, if a polyurethane formulation is designed to produce a foamed polyurethane having foam cells, whether filled with air or with a blowing agent, of a particular average diameter range, voids are any cells or holes that have diameters that are significantly larger than the desired average diameters. If the formulation is designed to produce a non-foamed polyurethane, then voids are any significant voids that result in a significantly non-uniform cross-section.
In view of the fact that any non-uniformity in the polyurethane structure risks undesirable performance reduction and/or, at the least, cosmetic detraction, those skilled in the art continue to seek means and methods of reducing the occurrence and/or size of such voids. Furthermore, it is recognized that reductions in the occurrence and/or size of voids may enable corresponding reductions in the amount or thickness of the polyurethanes used, enabling lower costs without reduced performance, or increased performance without correspondingly increased costs.